Mt. Airy is a small town at the geographical heart of central Maryland. It is the only place in the state where four counties meet, Frederick, Carroll, Howard and Montgomery. It is west of Baltimore and north of Washington. The headwaters of the Patapsco and Patuxent rivers are there only .6 of a mile apart.
The four counties meet at Parr's Spring, the headwater of the Patapsco on the historic Four Counties Farm on Parr's Ridge. The Patapsco river divides Carroll and Baltimore counties on the north and east from Frederick, Howard and Anne Arundel counties on the south and west as well as Baltimore City on the northeast from Anne Arundel county to the southewest The Patuxent river divides Howard, Anne Arundel and Calvert counties on the east from from Montgomery, Prince George's, Charles and St. Mary's counties on the west. The historic National Pike from Baltimore to Illinois passes along the southern boundary of Mt. Airy, as does the Old Main Line of the Baltimore and Ohio Railroad, now owned and operated by CSX.